Abstract

Numerous hydrologic and environmental models, with a range of different levels of approach and complexity, have been developed to strengthen and complement field studies. The Environmental Policy Impact Climate (EPIC) and the Agricultural Policy Environmental eXtender (APEX) models are flexible and dynamic tools that were developed to evaluate a wide array of management strategies applied to crop, pasture, and grazing lands. They are capable of estimating long-term sustainability of land management in respect to erosion (wind, sheet, and channel), economics, water supply, water quality, soil quality, plant competition, weather, and pests for cropland, as well as grazing and pasture land. This chapter describes the main features of the EPIC and APEX models and reports on the parameterization, calibration, and validation of the models. As a case study, the APEX model was applied to the North Bosque watershed in central Texas, where the major land use/land cover is native pastures/range and improved pasture, with dairies and milk sheds. Influential model parameters were adjusted during the model calibration period on the basis of the monthly stream flow, sediment yield, and nutrient losses at the Hico monitoring station. The model performance was reasonable, and this application shows that the APEX model is a powerful tool for simulating feedlot water quality.

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